This invention relates to an overload protection system for an electric power system, and in particular, to an improved arrangement for providing in combination current limiting regulation and overload cut off protection.
Certain electric load systems have variable electrical load requirements and must, under some conditions, operate with current levels approaching those at which equipment damage could result. For example, in precipitator systems AC input is commonly supplied through contacts of a circuit breaker and through a regulating arrangement, such as phase controlled silicon controlled rectifiers, to a transformer. The high voltage output of the transformer secondary is rectified and the rectified output is supplied to the separated wires and plates of the precipitator device. A control system controls the regulating arrangement to provide a proper potential across the precipitator. The precipitator current is thus maintained at the maximum value attainable without excessive sparking and without arcing. This maximum precipitator current varies due to many parameters and may result in current levels harmful to power circuit wiring and components.
Accordingly, it is desirable to limit power system current to a predetermined maximum value, by means of a current limit circuit which senses the magnitude of the current in a line of the power system. When the current attains a predetermined limit value, the current limit circuit actuates a voltage control regulator, such as phase controlled rectifiers, to maintain the power line current within the predetermined limit value. This is preferably accomplished by a current limit feedback loop incorporating a sensor of power line current, a current limit control system and the voltage control regulator in the power system.
In the event of malfunction, such as short circuits, the current limit regulating loop may be unable to maintain current within the predetermined current limit value. Therefore, an additional power interrupt arrangement is required. The power interruption system opens contacts, preferably at the inputs of the electric power circuit, responsive to abnormal overloads. In order to protect the lines and components within the power system, the overcurrent protective breaker operates to open the contacts with a time delay inversely related to the severity of the abnormality. In order to provide optimum circuit protection when fault or short circuit conditions develop, the operating characteristic of such an overcurrent cut off system should closely parallel an I.sup.2 t equals a constant relationship, that is, the operating time (t) of the cut off system should vary inversely in proportion to the square of the circuit current. Thus, the operating characteristic matches the damage characteristic of the protected circuit under fault conditions when the threat of damage is proportional to the current value squared.
The prior art has utilized separate current limit circuits and power interrupting, or cut off, arrangements of the type described above. The I.sup.2 t = k function of the power interruption circuit represents a hyperbolic current vs. time function curve, which for large time intervals is asymptotic with a load current line, termed 100%I, which constitutes the predetermined limit current value established by the current limit regulating circuit. However, due to normal tolerance variations, the I.sup.2 t = k function of the overload cut off circuit deviates from the 100 I.sub.p so as not to be asymptotic therewith. In order to prevent undesirable power interruption, i.e. tripping of the circuit breaker at current values below 100 percent I.sub.p, the I.sup.2 t=k characteristic was typically offset from the 100 percent I.sub.p line. This may however, result in excessive and potentially damaging current flow prior to the tripping of the breaker by the overload cut off circuit.